The present invention relates to a voltage stabilizer for stabilizing the output voltage of a high-voltage generating circuit supplied to a cathode-ray tube in a television receiver, display unit or the like.
Voltage stabilizers for a high-voltage generating circuit have been disclosed in Japanese Patent Laid-Open Nos. 149178/1981 (conventional example 1) and 140771/1981 (conventional example 2). According to voltage stabilizers disclosed in these publications, a high output voltage is divided by voltage-dividing resistors or the like, the divided high output voltage is compared with a reference voltage, and the power-source voltage for the high-voltage generating circuit is so controlled that the difference is minimized between the above two voltages, so that the high output voltage is stabilized.
FIG. 10 schematically shows the voltage stabilizer disclosed in the conventional example 1, which consists of a switching pulse input terminal 1, a switching transistor 2 for producing high voltage, a damper diode 3, a resonance capacitor 4, a fly-back transformer 5, a high-voltage rectifier diode 6, a high-voltage output terminal 7, resistors 8, 9, 16, 17, a detector transistor 10, a variable resistor 11, an error amplifier transistor 12, a Zener diode 13, an inverse current absorbing capacitor 14, a control transistor 15, and a power source 18.
Operation of the conventional voltage stabilizer will be described below.
The high voltage-producing transistor 2 shunts one end of a primary coil 5a of the fly-back transformer 5 responsive to switching pulses that are input to the switching pulse input terminal 1 and that have a period T.sub.H. Therefore, a pulse-like current flows in the primary coil 5a of the fly-back transformer 5, and a high voltage is induced in the secondary coil 5b. The rectifier diode 6 rectifies the high voltage and sends a high d-c voltage to the high-voltage output terminal 7.
The d-c high voltage is divided by the resistors 8, 9, and is applied to the base of error amplifier transistor 12 via detector transistor 10 and variable resistor 11. The detector transistor 10 must have a high input impedance; hence, it is used in the form of an emitter follower circuit. The emitter of the error amplifier transistor 12 is connected to a reference voltage source which consists of Zener diode 13 and resistor 17, and the collector thereof is connected to the base of control transistor 15.
Here, as the current driven by high voltage flows through the secondary coil 5b of the fly-back transformer 5 and the high output voltage appearing on the high-voltage output terminal 7 decreases, the base voltage of the detector transistor 10 decreases and the base voltage of the error amplifier transistor 12 decreases, too, causing the base voltage of the control transistor 15 to increase. Therefore, emitter voltage of the control transistor 15 increases, increased pulse-like current flows in the primary coil 5a of the fly-back transformer 5, and an increased voltage is produced by the secondary coil 5b.
As the voltage appearing on the high-voltage output terminal 7 increases, the voltage stabilizer operates inversely, whereby the base voltage of the control transistor 15 decreases, and the secondary coil 5b of the fly-back transistor 5 produces a decreased voltage.
According to this voltage stabilizer as described above, the voltage appearing on the high-voltage output terminal 7 is compared with the voltage of a reference voltage source which consists of resistor 17 and Zener diode 13, and the output voltage is so controlled as to be equal to the voltage of the reference voltage source at all times.
The high-voltage stabilizer of the type in which the power source is controlled, mentioned above, features stable operation but has the disadvantage of slow response speed. The reason for this disadvantage will be described below.
In FIG. 10, the inverse current absorbing capacitor 14 absorbs the inverse current I.sub.R that flows through the damper diode 3, so that the emitter voltage of the control transistor 15 is stabilized to assume a d-c voltage. The capacitor 14 must have a large capacity to sufficiently absorb the inverse current I.sub.R Therefore, the time constant for charge and discharge of the capacitor 14 increases, and the response time for the control operation increases.
If the capacity of the capacitor 14 is reduced to increase the response speed, the terminal voltage of the capacitor 14 assumes a parabolic waveform of a period T.sub.H, and the waveform of the control transistor 15 changes from the waveform shown in FIG. 11(a) into the wavefrom shown in FIG. 11(b). Namely, the emitter voltage 21' of the control transistor 15 rises and is cut off when it has exceeded the base voltage; i.e., switching operation is performed. In FIGS. 11(a) and 11(b), a line 19 represents a collector voltage, a line 20 represents a base voltage, and lines 21, 21' represent emitter voltages.
When the control transistor 15 is cut off, an inverse base current I.sub.bR flows out from the base thereof. Therefore, the base current of the control transistor 15 goes unstable and oscillates.
In order to prevent this oscillation, a method has been proposed as shown in FIG. 12 to stabilize the base voltage by providing a capacitor 14' which absorbs an inverse base current between the base of the control transistor 15 and ground. With this method, however, the capacitor 14' must have a large capacity. Hence, the time constant for charge and discharge becomes so large that it is difficult to quicken the response for control.
If the capacity of the inverse current-preventing capacitor 14 is further reduced, the peak value of the voltage of the parabolic waveform at the terminal of the capacitor 14 exceeds the emitter-base withstand voltage of the control transistor 15 to destroy the transistor.
Furthermore, if there is a portion where the brightness changes greatly on the screen of a cathode-ray tube which is connected to the high-voltage output terminal 7, the high-voltage output current that flows into the cathode-ray tube changes rapidly, and the voltage also changes at the high-voltage output terminal 7. In this case, however, control operation for the power-source voltage by the voltage stabilizer cannot sufficiently follow the above-mentioned change in voltage. Therefore, there develop such malfunctions as fluctuating screen amplitude, deviating convergence, loss of focusing, and the like.